The present invention pertains to a known type of sewing unit having a sewing machine with the usual stitching instrumentalities and means for transporting and advancing a workpiece along the worksurface of a support structure on which said machine is mounted. The unit includes a tensioning device in the form of a traveling gripper which grips the trailing end of a workpiece as it moves along the worksurface. Additionally, the unit includes a workpiece guide formed by a plurality of spaced plate elements disposed one above the other which is located in the path of travel of the workpiece intermediate the stitching instrumentalities and the tensioning device.
Sewing units of the type described are used primarily for the assembly of superimposed layers of fabric such for example as layers for forming trousers wherein the seams are not rectilinear, but follow a pattern determined by the cut of the fabric. This pattern is followed by guiding the separate pieces of fabric within the spaces formed between the plate elements of the workpiece guide. The trailing end of the workpiece is restrained by the traveling gripper having a movable jaw which can be opened by a lever operatively connected thereto. The traveling gripper is continually urged, by any suitable means such as a counterweight, in a direction opposite to the direction of advance of the workpiece.
Frequently workpieces have areas of increased thickness due to the addition of pockets or the like and for such workpieces it is well known to utilize a guiding device which is movable longitudinally along a portion of the path of travel of a workpiece. Such guiding devices are provided with means for gripping the workpiece and moving with the latter to a position of close proximity with the stitching instrumentalities. During actual seaming under these conditions the guiding device is attached to the workpiece at a location spaced from the stitching instrumentalities so that the area of increased thickness is located intermediate said guiding device and said stitching instrumentalities. As sewing continues, the guiding device is caused to be drawn by the workpiece to a position immediately adjacent to the stitching instrumentalities where it is caused to release its hold on said workpiece. In this position the guiding device continues to perform its guiding function on the workpiece until the tensioning device arrives at a position of close proximity with the upstream ends of the plate elements that form the passages in said guiding device.
Additionally, when the areas of increased thickness of a workpiece are located at a considerable distance from the starting point of the seam, it is well known to utilize two guiding devices with a first one being located adjacent the stitching instrumentalities and a second one movable with the workpiece as described above. With this arrangement the first guiding device adjacent the stitching instrumentalities is caused, by any suitable control apparatus, to be displaced from the path of travel of the second guiding device when the latter approaches the sewing zone so that said second device will assume the position of said first device. After the second guiding device takes over the position of the displaced first guiding device, it serves to continually guide the workpiece as described above until the tensioning device arrives at a position of close proximity with the upstream ends of the plate elements of said second guiding device.
The known types of sewing units briefly described above all have the disadvantage of not being able to maintain tension on the trailing portion of the workpiece having a length which corresponds to the distance between the upstream ends of the plate elements where said workpiece is released from the tensioning device and the stitching instrumentalities. This creates an undesirable condition when forming the rest of the seam in the remaining portion of the workpiece for it is likely to be unsatisfactorily incorporated. This is attributed to the fact that the intended function of the guide is lost in the absence of tension on the workpiece and unsatisfactory incorporation of the remaining portion of a seam is more pronounced with guides having plate elements of extra long length.